Ceramic-metal seals



United States Patent 47,320/64 U.S. 'Cl. 11770 7 Claims Int. Cl. B32]:17/06; B28b 19/00 ABSTRACT OF THE DISCLOSURE Ceramic articles have afired coating of a eutetic mixture of metal oxides having a meltingtemperature below that of the ceramic and including another metal of thegroup of nickel, cobalt and iron. The coating does not react chemicallywith the ceramic and is wettable by brazing alloys to provide a hermeticseal.

This invention relates to the metallizing of ceramic articles wherebythey may be brazed to metal members or to other similarly metallizedceramic parts. The invention finds particular application in themanufacture of hermetically sealed envelopes for electric dischargedevices and terminal plates for other electrical apparatus.

In application Ser. No. 444,100, filed Mar. 23, 1964, and assigned tothe same assignee as the instant invention, there is disclosed andclaimed a ceramic article having thereon a fired coating of a mixture ofmetal oxides having a definite melting temperature below that of anyconstituent of the ceramic and not forming a chemical reaction productwith the ceramic, the coating being loaded with a metallizing power suchthat the ceramic article may be hermetically sealed to a metal articleby brazing the metal article to the loaded coating or onto a layer ofanother metal plated on the coating. Preferably the mixture of metaloxides is a eutectic mixture, of which several examples include 92% ofCu O, 8% SiO 50% Li O, 50% SiO 82% SiO 18% Li O; 55% Cr O 45% C20; 52%CaO, 48% B 0 36% MgO, 64% B 0 and 25% SiO 65% MnO.

The choice of metallizing powder is likewise quite wide and includesmolybdenum and tungsten, nickel-chrome alloy, stainless steel, siliconcarbide and molybdenum disilicide. The examples of metallizing powdergiven, particularly molybdenum, provide metal to ceramic seals of highstrength. On the other hand molybdenum, which in common with allmetallizing powders quoted in the present specification, is not wetreadily by the usual brazing alloys such as copper-silver andcopper-gold. The fired coating has therefore to be nickel-plated beforebeing brazed to another article.

According to the present invention there is provided a ceramic articlehaving a fired coating wettable by brazing alloys wherein the firedcoating includes, at least on its outer surface, nickel, cobalt, oriron, or a mixture of these.

For some purposes, where a comparatively weak seal can be tolerated, thefired coating on the ceramic article may consist of a mixture of metaloxides, preferably a eutectic mixture, with metallizing powder ofnickel, cobalt or iron or a mixture of these. The strength of a typicalseal using iron is 8,000 lbs/inch compared to 14,500 lbs./in. for asimilar seal based on molybdenum, the stresses quoted being thoserequired to rupture a seal in a standard tensile testing machine. If thestronger seal is required, then we have found it satisfactory to preparea coating as described in the aforementioned parent specification but,before firing, to apply a second coating containing the metal or oxideof nickel, cobalt or iron or a mixture of these and then to fire thecomposite coating in a reducing atmosphere.

Examples of these two methods of manufacture are given below.

EXAMPLE 1 An iron powder mixture is made up as follows:

G. Iron powder (6,) Manganese dioxide (technical) 19 Silica powder(technical) 24.5 Alumina powder 10 This mixture is ball-milled with abinder of nitrocellulose in amyl-acetate for several hours until themaximum particle size is reduced to less than 10 The resultant paint isapplied to the ceramic to a thickness of 0.002 to 0.003 inch. Firing iscarried out at 1200 C. in wet hydrogen for 30 minutes. The fired coatingcan be brazed by conventional techniques to nickel-cobalt-iron alloyswithout further treatment.

For providing, for example, an outer layer of an iron group metal to themolybdenum powder coating given in the example of the parentspecification, the eutectic mixture given above may be usedbefore-firing or, alternatively, the outer coating may be applied by themethod given in the following example.

EXAMPLE II A molybdenum powder-loaded coating containing molybdenumpowder of 99.87% or greater purity, of particle size 15;t125 grams;Mn019 grams, silica powder--24.5 grams; high purity alumina-10 grams,with a binder as described above, is applied to a ceramic article and,before firing, an outer coating of a paint made up as follows isapplied:

Metal powder (iron, cobalt or nickel) 6/L-30 g.Nitrocellulose-amyl-acetate binder, 17 mls.

The molybdenum-based paint is, however, applied only to a thickness of0.0015 of an inch and the final layer of iron group paint is 0.0005 inchthick. The ceramic article is then fired in a hydrogen furnace at 1200C. The furnace is heated slowly and then maintained at the desiredtemperature above and close to the eutectic temperature for 30 minuteswith a continuous flow of wet hydrogen. The articles are then cooled toabout 200 C. before removal.

Instead of using metal powder for the coating, it is satisfactory to usemetal oxide, the paint then consisting, by way of example, of 20 g. ofmetal oxide dispersed in 7 mls. of binder. The metal oxide is reduced tothe metal during the hydrogen firing.

It is to be understood that the foregoing description of specificexamples of this invention is made by way of example only and is not tobe considered as a limitation on its scope.

We claim:

1. A ceramic article having a fired coating of a eutectic mixture ofmetal oxides selected from the group consisting of cuprous oxide,silica, lithium oxide, chromic sesquioxide, calcium oxide, boricanhydride, magnesium oxide, manganese oxide and alumina having adefinite melting temperature below that of any constituent of theceramic and which does not chemically react therewith, the coating beingfired at a temperature above and close to the eutectic temperature ofbetween 1000 to 1300 C. for a sufiicient time for said coating to becapable of forming a hermetic seal, said coating being wettable bybrazing alloys and including at least on its outer surface a metal ofthe group consisting of nickel, cobalt and iron.

2. A ceramic article as claimed in claim 1, wherein the coating includesa metallizing powder of said metal of the group consisting of nickel,cobalt and iron.

"3. A method of coating a ceramic article including applying a coatingof a eutectic mixture of metal oxides selected from the group consistingof cuprous oxide, silica, lithium oxide, chromic sesquioxide, calciumoxide, boric anhydride, magnesium oxide, manganese oxide and aluminahaving a definite melting temperature below that of any constituent ofthe ceramic and which when fired do not chemically react with saidceramic, said coating including a metal of the group consisting ofnickel, cobalt and iron and, firing said coating at a temperature closeto and above said eutectic temperature of between 1000 to 1300 C. in areducing atmosphere for a sufiicient time to obtain a coating capable ofhermetic sealing to metal articles by brazing.

4. The method of claim 3 including applying a first coating includingsaid mixture of metal oxides and a metallizing powder having molybdenumtherein and applying a second coating including said metal of said groupconsisting of nickel, cobalt and iron before firing.

5. The article of claim 1 wherein the coating includes a second metaloxide of said metal of the group consisting of nickel, cobalt and iron.

6. The method of claim 3 wherein said metal of said group is in the formof an oxide.

7. The method of claim 3 wherein said metal of said group is in the formof a metallizing powder.

References Cited UNITED STATES PATENTS 3,051,592 8/1962 Woerner 117-123XR 3,107,756 10/ 1963 Gallet.

3,132,044 5/ 1964 Pearsall 117123 XR 3,215,555 11/1965 Krey 117--123 XRFOREIGN PATENTS 1,134,922 8/ 1962 Germany.

ALFRED L. LEAVIIT, Primary Examiner.

20 W. F. CYRON, Assistant Examiner.

US. Cl. X.R. 11771, 123, 169

